1. Field of the Invention
The present invention relates to the determination of a spatial formation parameter such as slowing down length and porosity in a measurement while drilling ("MWD") tool. The invention, more particularly, relates to the use of a neutron source and multiple neutron detectors in an MWD tool to determine the slowing down length of neutrons in the formation and to derive the formation porosity from the determined slowing down length.
2. Description of the Prior Art
Prior art porosity logging tools have used neutron emission/detection devices to measure and/or calculate formation porosity. The presence of hydrogen in a formation is an indicia of formation porosity. Hydrogen is an effective moderator of neutrons. It is well known in the prior art that the degree to which neutrons emitted from a fast or epithermal neutron source are moderated is an indicia of formation porosity. The relationship between hydrogen content, neutron moderation, and porosity is discussed in R. F. Roesler et al., "Theory and Application of an MWD Neutron Porosity Sensor", Society of Petroleum Engineers of AIME, Paper No. 16057, presented at the 1987 SPE/IADC Conference in New Orleans, La.
Drilling fluid is made up substantially of hydrogen and therefore functions as a neutron moderator. It is well known in the prior art that the presence of drilling fluid in the borehole affects the accuracy of neutron logging measurements aimed at determining formation porosity. Since the hydrogen content of drilling fluid in a borehole is usually higher than the hydrogen content of a formation surrounding the borehole, neutron logging measurements made with prior art porosity logging devices provide a higher indication of porosity than is accurate for the formation.
It is well known in the art that environmental parameters such as stand-off, borehole diameter, borehole shape, and drilling fluid salinity affect the accuracy of neutron logging measurements aimed at determining formation porosity. The effects of these parameters are discussed in H. D. Scott et al., "Dual Porosity CNL* Counting Rate Processing", Society of Petroleum Engineers of AIME, Paper No. 11146 (1982). Prior art neutron logging porosity tools have attempted to correct for borehole environmental effects and tool stand-off using ratio techniques and/or crossplot techniques as disclosed in U.S. Pat. No. 4,760,252 to Albats et al.
A problem inherent in prior art porosity logging devices is the presence of errors in the porosity measurements due to the environmental parameters discussed above. Correction techniques employed in the prior art are attempts at compensating for the inaccuracies resulting from environmental effects such as borehole size and stand-off. These corrections or compensation techniques do not entirely eliminate the errors resulting from these environmental effects.
The present invention provides a method for determining the porosity of a formation surrounding a borehole that is independent of borehole size and stand-off. Porosity measurements taken with the method of the present invention are free from the inherent inaccuracies present in the prior art due to environmental effects such as borehole size and stand-off.